The hematopoietic system is often dose limiting for both chemotherapy and radiation therapy. It is the purpose of these investigations to understand the response of hematopoietic progenitor cells to these agents: further to see whether these can be modified to improve the therapeutic efficacy of cytotoxic agents. In these experiments we shall: 1) Study whether the stem cell has a limited proliferative capacity and if so, whether this is further limited after apparent recovery from cytotoxins; 2) Develop a repopulation assay for lymphocytes; 3) Study the mechanism of action of low dose whole body radiation.